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            <h2 class="title" style="clear: both"><a id="am_foreign"></a>Foreign key indexes</h2>
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      <p>
        Foreign keys are used to ensure a level of consistency
        between two different databases in terms of the keys that the
        databases use. In a foreign key relationship, one database is
        the <span class="emphasis"><em>constrained</em></span> database. This database
        is actually a secondary database which is associated with a
        primary database. The other database in this relationship is
        the <span class="emphasis"><em>foreign key</em></span> database. Once this
        relationship has been established between a constrained
        database and a foreign key database, then: 
    </p>
      <div class="orderedlist">
        <ol type="1">
          <li>
            <p>
                Key/data items cannot be added to the constrained
                database unless that same key already exists in the
                foreign key database.
            </p>
          </li>
          <li>
            <p>
                A key/data pair cannot be deleted from the foreign
                key database unless some action is also taken to keep
                the constrained database consistent with the foreign
                key database.
            </p>
          </li>
        </ol>
      </div>
      <p>
        Because the constrained database is a secondary database,
        by ensuring it's consistency with a foreign key database you
        are actually ensuring that a primary database (the one to
        which the secondary database is associated) is consistent with
        the foreign key database.
    </p>
      <p>
        Deletions of keys in the foreign key database affect the
        constrained database in one of three ways, as specified by the
        application:
    </p>
      <div class="itemizedlist">
        <ul type="disc">
          <li>
            <p>
                <code class="literal">Abort</code>
            </p>
            <p> 
                The deletion of a record from the foreign database
                will not proceed if that key exists in the constrained
                primary database. Transactions must be used to prevent
                the aborted delete from corrupting either of the
                databases.
            </p>
          </li>
          <li>
            <p>
                <code class="literal">Cascade</code>
            </p>
            <p> 
                The deletion of a record from the foreign database
                will also cause any records in the constrained primary
                database that use that key to also be automatically
                deleted. 
            </p>
          </li>
          <li>
            <p>
                <code class="literal">Nullify</code>
            </p>
            <p> 
                The deletion of a record from the foreign database
                will cause a user specified callback function to be
                called, in order to alter or nullify any records using
                that key in the constrained primary database. 
            </p>
          </li>
        </ul>
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      <p>
        Note that it is possible to delete a key from the
        constrained database, but not from the foreign key database.
        For this reason, if you want the keys used in both databases
        to be 100% accurate, then you will have to write code to
        ensure that when a key is removed from the constrained
        database, it is also removed from the foreign key database.
    </p>
      <p> 
        As an example of how foreign key indexes might be used,
        consider a database of customer information and a database of
        order information. A typical customer database would use a
        customer ID as the key and those keys would also appear in the
        order database. To ensure an order is not booked for a
        non-existent customer, the customer database can be associated
        with the order database as a foreign index. 
    </p>
      <p>
        In order to do this, you create a secondary index of the
        order database, which uses customer IDs as the key for its
        key/data pairs. This secondary index is, then, the constrained
        database. But because the secondary index is constrained, so
        too is the order database because the contents of the
        secondary index are programmatically tied to the contents of
        the order database.
    </p>
      <p> 
        The customer database, then, is the foreign key database.
        It is associated to the order database's secondary index using
        the <a href="../api_reference/C/dbassociate_foreign.html" class="olink">DB-&gt;associate_foreign()</a> method. In this way, an order cannot
        be added to the order database unless the customer ID already
        exists in the customer database.
    </p>
      <p> 
        Note that this relationship can also be configured to
        delete any outstanding orders for a customer when that
        customer is deleted from the customer database. 
    </p>
      <p>
        In SQL, this would be done by executing something like the
        following:
    </p>
      <pre class="programlisting">CREATE TABLE customers(cust_id CHAR(4) NOT NULL,
       lastname CHAR(15), firstname CHAR(15), PRIMARY KEY(cust_id));
CREATE TABLE orders(order_id CHAR(4) NOT NULL, order_num int NOT NULL,
       cust_id CHAR(4), PRIMARY KEY (order_id),
       FOREIGN KEY (cust_id) REFERENCES customers(cust_id) 
       ON DELETE CASCADE); </pre>
      <p>
        In Berkeley DB, this would work as follows:
    </p>
      <a id="prog_am18"></a>
      <pre class="programlisting">struct customer {
       char cust_id[4];
       char last_name[15];
       char first_name[15];
};
struct order {
       char order_id[4];
       int order_number;
       char cust_id[4];
};

....

void
foreign()
{
       DB *dbp, *sdbp, *fdbp;
       int ret;

       /* Open/create order database */
       if ((ret = db_create(&amp;dbp, dbenv, 0)) != 0)
               handle_error(ret);
       if ((ret = dbp-&gt;open(dbp, NULL,
           "orders.db", NULL, DB_BTREE, DB_CREATE, 0600)) != 0)
               handle_error(ret);

       /*
        * Open/create secondary index on customer id.  Note that it
        * supports duplicates because a customer may have multiple
        * orders.
        */
       if ((ret = db_create(&amp;sdbp, dbenv, 0)) != 0)
               handle_error(ret);
       if ((ret = sdbp-&gt;set_flags(sdbp, DB_DUP | DB_DUPSORT)) != 0)
               handle_error(ret);
       if ((ret = sdbp-&gt;open(sdbp, NULL, "orders_cust_ids.db",
           NULL, DB_BTREE, DB_CREATE, 0600)) != 0)
               handle_error(ret);

       /* Associate the secondary with the primary. */
       if ((ret = dbp-&gt;associate(dbp, NULL, sdbp, getcustid, 0)) != 0)
               handle_error(ret);

       /* Open/create customer database */
       if ((ret = db_create(&amp;fdbp, dbenv, 0)) != 0)
               handle_error(ret);
       if ((ret = fdbp-&gt;open(fdbp, NULL,
           "customers.db", NULL, DB_BTREE, DB_CREATE, 0600)) != 0)
               handle_error(ret);

       /* Associate the foreign with the secondary. */
       if ((ret = fdbp-&gt;associate_foreign(
           fdbp, sdbp, NULL, DB_FOREIGN_CASCADE)) != 0)
               handle_error(ret);

}

/*
* getcustid -- extracts a secondary key (the customer id) from a primary
*     key/data pair
*/
int
getcustid(secondary, pkey, pdata, skey)
       DB *secondary;
       const DBT *pkey, *pdata;
       DBT *skey;
{
       /*
        * Since the secondary key is a simple structure member of the
        * record, we don't have to do anything fancy to return it.  If
        * we have composite keys that need to be constructed from the
        * record, rather than simply pointing into it, then the user's
        * function might need to allocate space and copy data.  In
        * this case, the DB_DBT_APPMALLOC flag should be set in the
        * secondary key DBT.
        */
       memset(skey, 0, sizeof(DBT));
       skey-&gt;data = ((struct order *)pdata-&gt;data)-&gt;cust_id;
       skey-&gt;size = 4;
       return (0);
}</pre>
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